1. Field of the Invention
This invention relates to a process for preparing a positive-acting photosensitive lithographic printing plate precursor, and, more particularly, to a process for preparing a positive-acting photosensitive lithographic printing plate precursor by electrolytically graining an aluminum plate (including an aluminum alloy plate), etching the grained plate with an alkali, and anodizing the etched plate.
2. Description of the Prior Art
Lithography is a printing method that uses the intrinsic immiscibility of water and oil. On the surface of a printing plate, an area that receives water and repels oily ink (non-image area) and an area that repels water and receives oily ink are formed. The lithographic printing plate uses an aluminum support that carries the non-image area and is required to have a high degree of hydrophilicity and water retention and provide tight adhesion with a photosensitive layer to be placed on the plate. To achieve this purpose, the surface of the aluminum support is subjected to graining (that is, the formation of fine ridges and recesses). Methods of graining include mechanical graining, such as ball graining, brush graining, and wire graining, electrolytic graining, and combinations of mechanical graining and electrolytic graining, as described in Japanese Patent Application (OPI) No. 63902/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). Electrolytic graining or its combination with mechanical graining is preferred because it provides non-directional grain, and the surface obtained is uniformly grained and has high water retention.
The thus-grained aluminum surface is soft and is easily worn. Therefore, generally the aluminum plate is then anodized to provide an oxide coating on which the photosensitive layer is to be formed. The surface of the anodized aluminum plate is hard, has high wear resistance, good hydrophilicity and water retention and provides intimate contact with the photosensitive layer. However, the electrolytically grained surface may either have smut deposited thereon, or it may not be uniformly grained. For instance, if the electrolytically grained plate is immediately thereafter subjected to anodization, a black oxide coating is formed; not only does the coating reduce the aesthetic value of the plate but it also provides low or uneven sensitivity for the photosensitive layer to be formed on it. Furthermore, a developed plate has an image area that is hardly distinguishable from a non-image area, and this presents difficulties in plate finishing operations, such as deletion and image erasure, that are indispensable to a photomechanical process. In addition, the presence of smut causes a printing plate to have a very short press life if the aluminum plate is immediately anodized and is overlaid with a photosensitive layer.
In order to eliminate at least partially the above-described disadvantages, a method comprising electrolytic graining in an electrolyte of hydrochloric acid and then alkali etching has been proposed in Japanese Patent Publication No. 28123/73, and a method comprising electrolytic graining and then desmutting with sulfuric acid has been proposed in Japanese Patent Application (OPI) No. 12739/78.
According to the investigation by the present inventors, the desmutting with sulfuric acid used in the above latter method effectively removed the smut deposited on the electrolytically grained surface, but this method required a prolonged period of time for making uniform the uneven surface generated by th electrolytic graining treatment, etc. Further, when an aluminum plate was subjected to the electrolytic graining in an electrolyte of nitric acid and then to the desmutting treatment with sulfuric acid, a lithographic printing plate produced from the resulting support generated scumming in nonimage areas. On the other hand, it was found that the above alkali etching method was superior to the desmutting method with sulfuric acid, since smut was effectively removed due to high solubility of aluminum in an alkali etching solution and further the surface of aluminum was rendered uniform by the alkali etching. However, the electrolytic graining in hydrochloric acid and the subsequent alkali etching resulted in excessive smoothing of the grain structure and, thus, a lithographic printing plate produced from the resulting support showed a short press life due to insufficient adhesion between the support and the photosensitive layer provided thereon. In particular, when a negative acting composition comprising a light-sensitive diazo resin as described in Japanese Patent Publication No. 28123/73 (supra) was provided as a photosensitive layer on such a support, a marked decrease in the press life was observed.